1. Field of the Invention
The present invention relates to a combiner for being connected to the output stage of a Doherty amplifier and combining an output signal from a carrier amplifier and an output signal from a peak amplifier.
2. Description of the Related Art
Recently, cellular telephone base stations have incorporated transmission amplifiers for communicating with cellular telephones that are present in the areas covered by the base stations. The transmission amplifier is required to be adapted for an increased number of cellular telephones in the areas and an increased communication rate which results from and wider usage of data communications in the cellular telephone environment. Therefore, the transmission amplifier needs to reduce a gain distortion and a phase distortion of its output signal. The transmission amplifier is a circuit with maximum electric power consumption in the base station. Consequently, it is important for cellular telephone service providers to improve the amplifier efficiency of the transmission amplifier in order to meet demands for a reduction in the cost of consumed electric power and demands for a reduction in greenhouse gas emissions that have been posed on the cellular telephone service providers for environmental protection.
Generally, there is a trade-off between the gain and phase distortions of the output signal of an amplifier and the amplifier efficiency thereof. At present, efforts are being made to seek a technology for improving those characteristics comprehensively. Specifically, in order to improve the gain distortion and the phase distortion of the output signal of a transmission amplifier, the output signal of the transmission amplifier is partly monitored and digitally processed, and an input signal applied to the transmission amplifier is adjusted to reduce the distortions based on the digitally processed signal. Such a process is known as a so-called DPD (Digital Pre Distortion) technology.
Heretofore, a Doherty amplifier is widely used as the transmission power amplifier in cellular telephone base stations for improving the amplifier efficiency with which to amplify cellular telephone signals.
The Doherty amplifier comprises a carrier amplifier and a peak amplifier connected in parallel to each other. The carrier amplifier and the peak amplifier are controlled in operation to improve the amplifier efficiency with which to amplify a signal such as a cellular telephone signal wherein the difference between the average power and the peak power is large.
To meet an increase in the communication rate for cellular telephone and an increase in the number of cellular telephones accommodated in base station areas, it is necessary to control a distortion of the output signal of a transmission amplifier. For such distortion control, a circuit based on the DPD technology has been used.
Specifically, the output signal of a Doherty amplifier which is produced by combining the output signals of the carrier and peak amplifiers thereof is partly extracted and digitally processed, and the input signal applied to the Doherty amplifier is adjusted to reduce distortions of the output signal of the Doherty amplifier based on the digitally processed signal.
To perform the above process, the circuit based on the DPD technology includes a Doherty amplifier combiner for combining output signals of the carrier and peak amplifiers and a directional coupler having a coupling in the range from −20 dB to −40 dB, the directional coupler being connected between the Doherty amplifier combiner and an output antenna. The directional coupler produces an output signal which is supplied as an input signal to a digital processor.
One conventional Doherty amplifier combiner is disclosed in Japanese Laid-Open Patent Publication No. 2008-252215. The disclosed conventional Doherty amplifier combiner is a circuit connected to the output terminal of a Doherty amplifier, for combining output signals of the carrier and peak amplifiers thereof. Specifically, the disclosed conventional Doherty amplifier combiner is in the form of a chip including, in a dielectric substrate, a first λ/4 line connected between a combining point where the output signals of the carrier and peak amplifiers are combined and the carrier amplifier and a second λ/4 line connected between the combining point and an output terminal.
The signal which passes through the directional coupler is output from the Doherty amplifier and hence has a high power level ranging from several tens W to several hundreds W. However, the directional coupler loses a large amount of electric power because it causes a large power consumption loss.
Due to errors caused when a circuit board on which the Doherty amplifier combiner and the directional coupler are mounted is manufactured and errors when the Doherty amplifier combiner and the directional coupler are installed on the circuit board, the output signal from the directional coupler tends to change in level and phase, and hence the digital processor tends to fail to carry out a sufficient corrective process. In such a case, the amplifier efficiency of the Doherty amplifier has to be sacrificed in order to keep the distortions of the amplifier output signal at a prescribed level.
The DPD technology referred to above makes it possible for an amplifier having a simple AM/AM distortion (gain distortion) curve shown in FIG. 44 of the accompanying drawings to perform a highly advanced distortion correcting process. On the other hand, the Doherty amplifier has an AM/AM distortion curve including an inflection point Pb shown in FIG. 45 of the accompanying drawings because of a mechanism wherein the peak amplifier starts to operate after the carrier amplifier starts to be saturated. The inflection point Pb becomes greater as the peak amplifier starts to operate at a higher output level in order to increase the efficiency of the Doherty amplifier. Consequently, if the distortions are to be reduced to a value allowable for the amplifier, then the efficiency of the Doherty amplifier has to be sacrificed.